Hydrogen is produced as a co-product in various industrial processes. For example, hydrogen is produced as a co-product in the electrolysis of aqueous alkali metal halide brines to yield the corresponding alkali metal hydroxide, the halogen, and hydrogen. Hydrogen is also produced as a co-product in the electrolysis of aqueous alkali metal sulfates to yield the alkali metal hydroxide, oxygen, and hydrogen. Common to these industrial processes is the application of electrical power, from an external power supply, across an anode and a cathode of an electrolytic cell, to yield oxygen or halogen at the anode, the alkali metal hydroxide at the cathode, and hydrogen as a cathode co-product.
The electrolytic production of chlorine and aqueous sodium hydroxide is a major industry with domestic production of hundreds of thousand tons per day of chlorine and a like amount of aqueous sodium hydroxide (anhydrous basis) at a power consumption of from 2,000 to 3,000 kilowatt hours per ton of chlorine or of aqueous caustic soda (anhydrous basis).
Similarly, hydrogen is produced as a means of storing energy by the electrolytic decomposition of hydrogen compounds such as water, and the recombination thereof in a fuel cell.
Common to all of these industrial processes is the application of large amounts of external power, power generated by fossil fuels or nuclear reactors.